The present invention is directed to a motor vehicle air conditioning system including a variable capacity refrigerant compressor driven by the vehicle engine, and more particularly to a control method for overriding the normal capacity control for the purpose of reducing engine exhaust emissions.
It has been shown that in a motor vehicle equipped with an engine-driven air conditioning compressor, normal operation of the compressor can significantly increase the engine exhaust emissions (particularly carbon-monoxide and oxides of nitrogen) during periods of vehicle acceleration. While it would be possible to simply de-couple the compressor from the engine during vehicle acceleration, as is commonly done during wide-open throttle engine operation, the occupant comfort would be significantly and unnecessarily degraded under many conditions. Accordingly, what is needed is a method of controlling the load imposed by an engine-driven compressor so as to reduce engine exhaust emissions without unnecessarily degrading occupant comfort.
The present invention is directed to an improved method of operation for an engine-driven air conditioning compressor, wherein the normal capacity control of a variable capacity air conditioning compressor is selectively overridden during periods of vehicle acceleration to effectively reduce engine exhaust emissions. According to the invention, the capacity override is invoked when the rate of engine throttle movement exceeds a predefined rate, provided that the load imposed by the compressor is sufficiently high and the vehicle speed is greater than a minimum value. Once invoked, the override reduces the compressor capacity to a predetermined level, and measures the elapsed time. The override is maintained for at least a minimum time period that ensures an emission benefit, after which the vehicle speed is monitored to determine the extent of the acceleration. The override is terminated when the rate of increase in vehicle speed falls below a reference rate, or when the elapsed time exceeds a reference time, whichever occurs first.